Various apparatus are available for amplifying signals. In amplifier applications that involve the amplification and transmission of modulated signals, a premium is placed on amplifier efficiency. In addition, because many applications require a wide bandwidth, a premium is placed on the ability to efficiently create a high fidelity reproduction of a wideband signal.
Communications devices, which often transmit wideband signals, are an example of an application where these qualities are in demand. Low distortion allows the communications devices to communicate more reliably, and high efficiency allows the devices to operate longer on a single battery.
Wideband communications signals usually have correspondingly wide modulation bandwidths. That is, when a signal occupies a large RF bandwidth, the envelope of the signal within that bandwidth is rapidly changing. An amplifier that efficiently amplifies a signal of this type preferably has a wide RF bandwidth and a wide modulation bandwidth.
One method of achieving increased efficiency is to use envelope elimination and restoration (EER)-type amplifiers. EER is a technique through which highly efficient but nonlinear radio frequency (RF) power amplifiers can be combined with other, highly efficient amplifiers to produce a high efficiency linear amplifier system. The signal to be amplified is split into two paths: an amplitude path, and a phase path. The detected envelope is amplified efficiently in the amplitude path by a class S or other highly efficient power amplifier which operates on the bandwidth of the RF envelope rather than the RF bandwidth. The phase modulated carrier in the phase path is then amplitude modulated by the amplified envelope signal, creating an amplified replica of the input signal.
In EER-type amplifiers the envelope signal occupying the modulation bandwidth is amplified in the amplitude path. Conventional EER-type amplifiers utilize class S modulators to amplify the modulation bandwidth included in the envelope of the input signal. Unfortunately, class S modulators are limited in bandwidth by the switching frequency at which they operate, and as the switching frequency increases, the class S modulator becomes less efficient. This practical constraint sets a maximum modulation bandwidth achievable for any given amplifier efficiency.
Accordingly, a need exists for a power amplifier that efficiently amplifies a wideband RF signal exhibiting a wide modulation bandwidth.